How to send YouTube videos from your Android app to a Chromecast device: https://medium.com/@soffritti.pierfrancesco/how-to-send-youtube-videos-from-your-android-app-to-a-chromecast-device-541d59ea1260

CastOptionsProvider.java

public final class CastOptionsProvider implements OptionsProvider {
  public CastOptions getCastOptions(Context appContext) {
    
    // Register your receiver on Cast Developer Console to get this ID: https://cast.google.com/publish
    String receiverId = "";
  
    return new CastOptions.Builder()
      .setReceiverApplicationId(receiverId)
      .build();
  }
  
  public List<SessionProvider> getAdditionalSessionProviders(Context context) {
    return null;
  }
}